It does everything Markdown can and much more. And it's older.

Emacs isn't everyone's cup of tea, but it was a mistake by Github to latch onto Markdown. Now we are stuck with such a limited markup format with so many incompatible versions--none of which are as adept as Org syntax.

I'm guessing it's because Org markup was not embraced early enough by non-Emacs users that Github didn't do it.